Vehicle assembly lines for automobiles, particularly those lines where parts, such as a power unit comprised of an engine and a transmission, and a suspension device, are installed in the bodies of the automobiles, have adopted a method in which the parts are transferred on a carriage, while each of the bodies is transferred by an overhead hanger, and these transfer movements are synchronized so that the parts are lifted and installed in the body from below.
This method uses a self-propelled carriage having a driving system, and this carriage is self-propelled along a travel path laid on the floor. In general, the travel path of the self-propelled carriage is a looped path which starts from a part mounting area, passes through a part installation work area along the movement path of the hanger, and returns to the part mounting station. The self-propelled carriage and the hanger need to be mechanically or electrically synchronized in the part installation work area, as explained below.
In the mechanically-synchronized technique, as disclosed in Patent Document 1, the synchronization is achieved by mechanically coupling the carriage to a hanger hanging down from the overhead conveyor, via a cramp unit. In the electrically-synchronized technique, on the other hand, the synchronization is achieved by detecting a relative position of the self-propelled carriage to the hanger, and controlling the speed of the carriage.
Patent Document 2 discloses a self-propelled carriage comprised of a so-called automated guided vehicle (AGV) having a driving system, and a jig vehicle on which parts are mounted. These vehicles can be coupled to, and separated from, each other. The AGV and the jig vehicle are moved to near the upstream end of the conveyor-type transfer system, while being coupled to each other, and are separated from each other at this point, where the jig vehicle is engaged with the transfer system which will transfer the jig vehicle. In the course of transfer, the parts mounted on the jig vehicle are assembled into an assembly product. The AGV, on the other hand, is moved to near the downstream end of the transfer system earlier than the jig vehicle, and waits for the jig vehicle, on which the completed assembly product is now mounted, to arrive there. At this point, the AGV and this jig vehicle are coupled to each other again.